Perhaps I should revise my statement. It isn't magit itself that's slow, but rather magit is calling out to slow git commands. And while those git commands are taking up 100% of my CPU, Emacs itself is using 0% CPU and totally unresponsive.

Just a few moments ago I had to kill Emacs because magit was executing this command:

    git branch --remote --contains 10a1663bf96f